Earth observing radiometers typically view the planet in separate solar (0.3-3 μm) and infra-red (3-100 μm) spectral channels. The solar channels measure the amount of sunlight reflected by the Earth, while the infra-red channels measure the thermal radiance emitted out to space. For the infra-red channels, onboard targets, like blackbodies, are used to calibrate the sensors. For the solar channels, typically a separate target, like a solar diffuser or an onboard lamp, is used to calibrate the sensors.
These Earth observing radiometers, or imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Satellite-based, remote sensors are able to measure properties of the atmosphere above the Earth, when their detector arrays are properly calibrated for radiometric response.
A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral radiance, such as the sun. The radiation from the sun is used as a reference signal which, in turn, provides a known radiance to a remote sensor for calibrating its detector array.
Conventionally, there are four different methods for calibrating the radiance measured by the sensors. One of these methods is the use of Calibration Light Source Assemblies (hereinafter “CLSA”). CLSAs use the sun as a source of illumination and provide a partial aperture illumination. A second method is the use of a Full Aperture Calibration Door (hereinafter “FACD”). An FACD provides a coating on the inner surface of a calibration door. During the calibration process, the door is opened, as shown in FIG. 1, and the coated inner surface reflects the sun towards the detector array of the radiometer. The sensor in FIG. 1 is represented schematically by a single lens 12 focusing an image onto a detector array 14. The sensor is calibrated by swinging a diffusely reflecting surface of a door 10, often a Lambertian surface, into a blocking position over the sensor aperture 16. The sun must be at a large angle to the sensor's line of sight, with solar radiation passing across the aperture, so that the Lambertian surface can fill the sensor's field of view (FOV) with diffusely reflected sunlight. Between calibration cycles the Lambertian surface must swing aside, clearing the line of sight, as shown by the arrow and dashed lines in FIG. 1.
Another method is the use of a Full-Aperture Calibration Surface (hereinafter “FACS”). In the FACS method, a medium or a coating is applied to a surface. The surface is then moved into position to reflect the sun as a source of illumination. Finally, another method that may be employed uses an On-board Calibration Source (hereinafter “OBCS”). In the OBCS method, incandescent lamps, or radiative black bodies are used to provide the illumination. The lamps or radiative black bodies are positioned in front of an aperture to a sensor such that they illuminate direct energy towards the sensor when calibration is required.
A satellite known as the Long Duration Exposure Facility (LDEF) has been recovered by the Space Shuttle after spending six years in low Earth orbit. The LDEF includes a telescope that has monitored solar radiation from the Earth. The telescope has been calibrated, as necessary, using an onboard tungsten lamp. FIG. 2 shows the change in spectral transmission of the LDEF telescope after recovery compared to before launch (spending 6 years in low Earth orbit).
Referring to the figure, three plots are shown of a ratio-to-witness samples (y-axis) versus wavelength (x-axis). One plot is for the change in the viewing characteristics of the LDEF telescope after 6 years in orbit; and another plot is for the change in the radiance characteristics of the onboard tungsten lamp after 6 years in orbit. A third plot is for the scattered radiance characteristics of a clear ocean. The figure shows that a telescope in low Earth orbit degrades primarily in the UV to blue spectral region; the degradation increases significantly at shorter wavelengths below 0.4 μm. The figure also shows that a tungsten lamp degrades significantly above 0.4 μm. It will also be appreciated that, as shown, the radiance scattered from a clear ocean (blue spectrum of sunlight) varies significantly as a function of wavelength.
The LDEF results shown in FIG. 2 lead to a conclusion that conventional onboard calibration sources, such as the lamps on board the Earth observing satellites of CERES and CLARREO have little to no calibrated standards that may be traceable to a NIST (National Institute of Standards and Technology) standard. This is especially true for radiance at visible wavelengths, as the radiance of the tungsten lamp shown in FIG. 2 has no spectral resolution below 0.4 μm. Conventional Earth observing solar channels tend to degrade in orbit with no means of monitoring the instrument changes. Thus, any biases and trends in both environmental and climate data records (EDRs and CDRs) released to the science community are incorrect; the trends represent instrument changes, rather than Earth climate changes.
As will be described, unlike the aforementioned deficiencies, the present invention provides an accurate system and method for calibrating a metrological instrument, such as an Earth viewing telescope, while it is orbiting the Earth.